1. Field of the Invention
This invention is generally related to the manufacture of contact lenses, and more specifically, to a new process for manufacturing "buttons" from which soft contact lenses having greater tear resistance can be produced.
2. Description of the Related Art
As is known in the art, contact lenses are frequently made of polymethyl methacrylate. Such lenses are known as "hard lenses". Many people have trouble adapting to the presence of a hard lens in their eye. Furthermore, these hard lenses often compromise the physiological processes required for corneal metabolism. For many, minor irritations may be caused by small particles that become lodged under the hard lens, resulting in the irritation of the cornea. Moreover, it is found that after wearing a hard lens for an extended time, e.g., from one to five years, many people experience chronic discomfort and are forced to discontinue its use.
In view of the above difficulties, "soft" or "hydrophilic" lenses were developed. Soft lenses are made of various hydrogel materials to reduce the irritation of the corneal surface. In addition, since these hydrophilic lenses are made of materials which have a higher water content, on the order of 50% to 65%, and tend to be physically thinner than the hard lenses, they tend to be more O.sub.2 permeable, allowing the user to wear the contact lens for extended periods of time. One specific hydrogel formulation disclosed in the earlier patents includes a predominant quantity of 2-hydroxyethyl methacrylate. This hydrogel, known as "HEMA", is used for contact lenses, in part, because of its ability to absorb water of hydration, typically from about 35% to about 65% by weight. The water renders the lens flexible and soft, which enables it to mold to the curvature of the eye. Various formulations of hydrogels have been disclosed which achieve a particular expansion coefficient, maximize optical transparency and exhibit desired physical stability characteristics.
The hydrogel is commonly polymerized into the form of small circular buttons, which are subsequently ground to prescription while still in their hard, unhydrated state. While the copolymerization process inherently calls for elevated temperatures, it is high temperature for extended periods of time that can have an adverse effect on the resulting product by causing certain essential volatile components to evaporate from the formulation. In order to reduce this loss, two different approaches have been taken in the prior art. One method calls for reducing the time required for polymerization by subjecting the monomers in an open mold to an even higher temperature for a short period of time. Subjecting the hardening button to the augmented thermal stress associated with this technique, however, reduces the tensile strength or tear resistance of the resulting product. Alternatively, the prior art suggests using closed molds to contain the monomers, thus allowing lower temperatures to be applied for a longer period of time. Although this minimizes the evaporation of the volatile constituents, the hardening button is exposed to stresses and strains due to its constraint within the closed mold. Either method results in a button that is of reduced tensile strength. A subsequent annealing step, in which the polymerized button is subjected to elevated temperature serves to recoup some of the strength loss by relieving internal stress, but the ultimate product's tensile strength remains nonetheless compromised.
Hence, those concerned with the manufacture of hydrogels for use in soft contact lenses have long recognized the need for a method of making the same which provides a resulting end product which is more tear resistant. Furthermore, there is a need for a method of making soft contact lenses which reduces the stress applied or created within the buttons during the manufacturing process and, at the same time, reduce the evaporation of volatile constituents at the surface or interface of such buttons. The present invention clearly fulfills all of these needs.